1. Field of the Invention
The present invention relates to a method of manufacturing a liquid crystal display device (LCD), and particularly to a method of manufacturing an active matrix type liquid crystal display device using a semiconductor thin film (hereinafter referred to as an AM-LCD). The present invention can be applied to an electrooptical device equipped with such a display device.
Incidentally, in the present specification, the term “semiconductor device” indicates all devices which function by using a semiconductor. Thus, the foregoing display device and the electrooptical device are included in the category of the semiconductor device. However, in the present specification, for facilitation of the distinction, terms such as a display device and an electrooptical device are selectively used.
2. Description of the Related Art
In recent years, a projector or the like using the AM-LCD as a projection type display has been vigorously developed. Moreover, the demand of the AM-LCD as a direct viewing display for a mobile computer or a video camera is also increasing.
FIGS. 2A and 2B schematically shows a structure of a pixel matrix circuit in a conventional active matrix type display device. Incidentally, the pixel matrix circuit is such a circuit that thin film transistors (TFTs) for controlling an electric field applied to a liquid crystal are arranged in matrix, and constitutes a picture image display region of the AM-LCD.
FIG. 2A is a top view showing the pixel matrix circuit seen from the above. Here, regions surrounded by a plurality of gate lines 201 provided in the horizontal direction and a plurality of source lines 202  provided in the vertical direction become pixel regions. A TFT 203 is formed at each of the intersections of the plurality of gate lines 201 and the plurality of source lines 202. A pixel electrode 204 is connected to each of the TFTs.
Thus, the pixel matrix circuit is constituted by the plurality of pixel regions formed in matrix by being surrounded by the plurality of gate lines 201 and the plurality of source lines 202, and the TFT 203 and the pixel electrode 204 are provided at each of the pixel regions.
FIG. 2B shows the structure of a section of the pixel matrix circuit. In FIG. 2B, reference numeral 205 denotes a substrate having an insulating surface, 206 and 207 denote pixel TFTs formed on the substrate 205, which correspond to the TFTs 203 shown in FIG. 2A.
The pixel TFTs 206 and 207 are connected to pixel electrodes 208 and 209, respectively. The pixel electrodes 208 and 209 correspond to the pixel electrodes 204 shown in FIG. 2A. The pixel electrodes 208 and 209 are generally obtained by patterning one metal thin film.
Thus, in the pixel matrix circuit of the conventional structure, boundary portions of electrodes (hereinafter simply referred to as boundary portions) 210 and 211 always exist between the pixel electrodes. That is, a difference in level, which corresponds to the film thickness of the pixel electrode, is inevitably formed. Poor orientation of a liquid crystal material occurs at such a difference in level, so that a displayed picture is disturbed. Besides, diffused reflection of incident light at the portion of the difference in level causes the contrast to lower or the efficiency of utility of light to lower.
As is apparent from FIG. 2B, the pixel electrodes 208 and 209 formed over the semiconductor elements or the intersections of the respective wiring lines have the state reflecting the shape of the semiconductor elements and the intersections. Such a difference in level also causes the foregoing problems.
Particularly, in a projection type display used for a projector or the like, since an extremely minute small display of about 1 to 2 inches in size is enlarged and projected, the foregoing problems become tangible.
In regard to the above described problems, a black mask (or a black matrix) has been conventionally used to shade the region where a picture image is disturbed, so that the ratio of contrast is increased. In recent years, since miniaturization of a device has been progressed and the controllability of a shading region for the purpose of a high aperture factor has been required, the structure in which the black mask is provided at a TFT side substrate has become the main stream.
However, in the case where the black mask is provided at the TFT side substrate, there arises various problems such as increase of patterning steps, increase of parasitic capacitance, and lowering of an aperture factor. Because of such circumstances, it is desired to achieve a technique by which the ratio of contrast can be assured without causing the above described problems.